1. Field of Invention
Present invention relates to a coordinate input apparatus which detects input vibrations and specifies the position of the vibration source so as to determine the coordinates of the vibration input position.
2. Related Art
Conventional coordinate input apparatuses utilizing ultrasonic vibration detect vibration inputted from a vibrating pen by a plurality of sensors on a vibration transmitting panel. The coordinates at the vibrating pen input position on the vibration transmitting panel is determined based on difference data, obtained from delay times at the respective sensors, calculated using a reference delay time from a point where the vibration is inputted to a point where one of the sensors (e.g., the sensor that first receives the vibration), as a reference sensor, detects the vibration.
As shown in FIG. 11, the above coordinate input apparatus calculates coordinates using the following equations in accordance with the positional relation between vibration sensors S1 to S4 on the vibration transmitting panel and a vibration input point P (x, y) : .DELTA.db-.DELTA.dd calculated from the difference data among the delay times are: EQU .DELTA.db=db-da (101) EQU .DELTA.dc=dc-da (102) EQU .DELTA.dd=dd-da (103)
Point P(x, y) is obtained from: ##EQU1##
Note that the equations (104) and (105) hold if: EQU .DELTA.db+.DELTA.dd-dc.noteq.0 (106)
If EQU .DELTA.db+.DELTA.dd-.DELTA.dc=0 (107)
the point P (x, y) is: ##EQU2## Otherwise, the point P(x, y) is: ##EQU3##
However, the above conventional coordinate determination method is implemented by simply assigning a start and stop signals of a counter to the respective signals, and measuring the time difference. When a plurality of signals are detected upon obtaining the difference data, the relation between group delay time and phase delay time at each sensor is reversed, causing erroneous judgment of the reference delay time.
If two counters are used, the size of the counters must be greater than the repetition period of a signal source. This increases the apparatus construction cost.
Further, the above coordinate calculation method has a drawback in that if the equation (106) holds, the results from the equations (104) and (105) tend to cause errors in the equations (108) and (111).
Generally, the conventional apparatus of this type measures time from a point where vibration generated by the vibrating pen is inputted to a point where the vibration reaches one of the vibration sensors provided at predetermined positions on the vibration transmitting panel, and calculates the distances between the vibrating pen and the vibration sensors, i.e., coordinates designated by the vibration pen, based on the measured values.
Especially, in a wireless type apparatus in which a vibrating pen and a coordinate input apparatus main body are not connected with a cord such as a power supply line and a signal line, the vibrating pen itself incorporates a power source, a vibrator and a vibrator driver.
In a case where this vibrating pen uses a power source such as a battery, the pen does not work as the power source is depleted and the power source must be exchanged. Preferably, in case of malfunction of the vibrating pen, whether the apparatus main body is out of order or the power source is depleted can be easily judged, more specifically, the pen itself has a function to inform a user of the deterioration of the power source. However, assuming that the pen has a display function to indicate residual power using, e.g., an LCD, as a rechargeable shaver, the number of electric parts to be incorporated in the pen increases. This enlarges the size of the vibrating pen, and further, raises consumption requirement of the power source. Even if a device for notifying the power-source state to the apparatus main body is provided, and the apparatus main body displays the informed power-source state, a similar problem may occur.